Abstract
Robust estimation of soil moisture using microwave remote sensing depends on extensive ground sampling for calibration and validation of the data. Soil surface sealing is a frequent phenomenon in dry environments. It modulates soil moisture close to the soil surface and, thus, has the potential to affect the retrieval of soil moisture from microwave remote sensing and the validation of these data based on ground observations. We addressed this issue using a physically-based modeling approach that accounts explicitly for surface sealing at the hillslope scale. Simulated mean soil moisture at the respective layers corresponding to both the ground validation probe and the radar beam’s typical effective penetration depth were considered. A cyclic pattern was found in which, as compared to an unsealed profile, the seal layer intensifies the bias in validation during rainfall events and substantially reduces it during subsequent drying periods. The analysis of this cyclic pattern showed that, accounting for soil moisture dynamics at the soil surface, the optimal time for soil sampling following a rainfall event is a few hours in the case of an unsealed system and a few days in the case of a sealed one. Surface sealing was found to increase the temporal stability of soil moisture. In both sealed and unsealed systems, the greatest temporal stability was observed at positions with moderate slope inclination. Soil porosity was the best predictor of soil moisture temporal stability, indicating that prior knowledge regarding the soil texture distribution is crucial for the application of remote sensing validation schemes.
Highlights
Soil moisture at the surface vicinity affects the partitioning of rainfall into infiltration and runoff and modulates soil-atmosphere feedback interactions [1,2]
It affects groundwater recharge, the movement of contaminants and crop growth [1], and it has been the focus of many studies that have addressed soil moisture dynamics across different climatic regions and spatial and temporal scales [3,4,5,6]
We addressed two questions: (1) How does the presence of a seal layer affect soil moisture at the top soil? What are the implications for the validation of close range and remotely sensed data?
Summary
Soil moisture at the surface vicinity affects the partitioning of rainfall into infiltration and runoff and modulates soil-atmosphere feedback interactions [1,2]. It affects groundwater recharge, the movement of contaminants and crop growth [1], and it has been the focus of many studies that have addressed soil moisture dynamics across different climatic regions and spatial and temporal scales [3,4,5,6]. The effective penetration depth of the microwave beam depends upon several factors, including soil moisture [14] and soil properties [15]. A common need for all remote-sensing (RS) missions, regardless of the spatial scale of interest, is intensive ground soil moisture sampling for the calibration of retrieval algorithms and the validation of the data [18]
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